Since its discovery in 1911, many applications for the phenomena of superconductivity have been conceived which could not be commercialized because of the extreme low temperatures required by the superconductive material. Although many materials have been examined since 1911 in an effort to find compounds which will superconduct at higher, more practical temperatures, the highest temperature superconductor known until about 1986 was Nb.sub.3 Ge having a critical temperature, To, of approximately 23.3.degree. K. Superconducting devices utilizing Nb.sub.3 Ge as the superconductor thus required the use of liquid helium as refrigerant-coolant in commercial applications.
In 1986 Bednorz and Muller disclosed that certain mixed phase compositions of La-Ba-Cu-O appeared to exhibit superconductivity at about 30.degree. K. Investigation of that system established that the crystalline phase therein responsible for superconductivity had a crystal structure like that of K.sub.2 NiF.sub.4 (214). The upper temperature limit of onset, T.sub.co, for superconductors of a 214 type crystalline structure has been found to be about 48.degree. K.
Following the discovery of superconductivity in such rare earth-alkaline earth Cu oxide systems of a 214 crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which were superconductive at temperatures above 77.degree. K.
This new class of rare earth-alkaline earth-copper oxides are now commonly referred to as "123" high-temperature superconductors. The "123" high temperature superconductors have perovskite related crystalline structures and a unit cell formula of L.sub.1 M.sub.2 Cu.sub.3 O.sub.6 +.delta.(.delta.=0.1 to 1.0, preferably about 1.0) wherein L is Scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and M is barium, strontium or mixtures thereof. The preferred 123 is Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 +.delta.. With the discovery of the 123 class of high temperature superconductivity compositions it has become possible to economically pursue many previously conceived applications of the superconductivity phenomena which were commercially impractical wherein cooling by liquid helium was required. Since they superconduct at temperatures greater than 77.degree. K., the new "123" class of high temperature superconductors may in practical applications be cooled with liquid nitrogen--a more economically feasible refrigerant. As a result, the rather complex thermal insulation and helium-recycling systems employed with conventional superconductors to avoid wasting the expensive helium coolant has been abandoned, thereby greatly simplifying and enhancing the reliability of commercial superconductors.
However, the prior applications of the heretofore high temperature superconductors have not been sufficient due to their inability to carry high current loads in intense magnetic fields, which thereby constitutes a significant commercial barrier against use of the 123 superconductors in numerous applications, such as in magnetic separators, transmission lines and magnetically levitating trains (meglav). In magnetic separators, for example, superconductors are required to have a current density, J, between about 33,000 and 66,000 amps/cm.sup.2 in a magnetic field between 2 and 3 T. Further, to be commercially viable, underground superconducting transmission lines cooled with liquid nitrogen must have the capacity to carry large amounts of current, approximately 10,000 to 40,000 amps/cm.sup.2 at a magnetic field of approximately 0.2 T.